1. Field of the Invention
The present invention generally relates to an electrolytic process for electrolysis of an aqueous alkali metal halide solution, especially an aqueous alkali metal chloride solution. More particularly it relates to a process for obtaining a high quality caustic alkali more effectively with low cell voltage using a horizontal type electrolytic cell providing a cation exchange membrane as an electrolytic separator.
2. Description of Prior Art
The most typical horizontal electrolytic cell is a mercury electrolytic cell. However, use of such a cell will be discontinued in the near future in Japan since mercury which is used as a cathode contaminates the environment. To convert a mercury cathode electrolytic cell into a separator electrolytic cell which does not employ mercury, with a reduced cost, the separator electrolytic cell should be of a horizontal type. In view of this situation, a significant matter the industry is now encountering is to develop a process for producing a high quality product, which is not inferior to the product of the mercury process, with a high current efficiency using horizontal type separator electrolytic cells.
A process for retrofitting a mercury cell to a horizontal type separator cell is revealed in the U.S. Pat. No. 3,923,614. In the process, a porous membrane (diaphragm) is used to serve as a separator. However, such a porous membrane has great water permeability and accordingly anolyte solution passes through the separator hydraulically and mixes with, for example, caustic alkali produced in the cathode compartment. This results in decreased quality.
On the other hand, a cation exchange membrane called a non-porous membrane permits no passage of anolyte solution or catholyte liquor hydraulically. This membrane allows only water molecules coordination-bonded to alkali metal ions transported electrically to pass. Hence a high quality caustic alkali is obtained. However, a small quantity of water transported evaporates causing electric conduction failure between a membrane and a cathode therefor. This, in the long run, terminates the electrolytic reaction.
The U.S. Pat. No. 3,901,774 proposes processes to solve these problems. One is a process for placing a liquid maintaining material between a cation exchange membrane and a cathode and another is a process for carrying out the electrolysis while supplying to a cathode an aqueous caustic alkali liquor in mist or spray.
The former process involves problems including trouble in interposing the liquid maintaining material and the durability therefor of the material. The process also increases cell voltage because the distance between electrodes is expanded by the liquid maintaining material located between the cation exchange membrane and the cathode. Also, there is an increase in electric resistance of the liquid maintaining material per se. Hence it can not be an advantageous process. Moreover the latter process has some difficulties in practice on an industrial scale since it is difficult to provide a uniform supply of liquid when the process is applied to a large-scale electrolytic cell as employed commercially.
Further, in effecting electrolysis using a horizontal electrolytic cell providing a cation exchange membrane positioned substantially horizontal, it is important to prevent cathode gas, generated on a cathode, from residing on the underside of the membrane, i.e., to keep the underside of the membrane in contact with catholyte liquor. Convensionally, the cation exchange membrane has been used in a vertical type electrolytic cell. In such a case, for the purpose of rapidly removing cathode gas evolved on the cathode from a space between the cathode and the cation exchange membrane, a perforated cathode having an aperture of from 90 to 10% and formed from materials such as expanded metal sheets, punched metal sheets, nets, louver-like cathodes is used. The evolved gas is removed behind the cathode. On the other hand, in the case of the horizontal type electrolytic cell, it is impossible to cause gas evolved on the cathode below the cation exchange membrane to discharge behind the cathode, i.e., underneath the cathode against the buoyancy. Therefore, the space between the cathode and the membrane is filled with cathode gas which impedes electric conduction.
In order to eliminate the foregoing defects, a process has been considered in which catholyte liquor is circulated in the space between the cathode and the membrane to remove evolved gas together with the circulated catholyte liquor from the cathode compartment. With the conventional perforated cathodes, however, the circulated catholyte liquor is dispersed underneath the perforated cathode so that residence of gas in the space between the cathode and the membrane can not be prevented perfectly. As a result, part of gas remains to thus result in an increase in cell voltage.